Franz I of Liechtenstein was 76 when he succeeded his brother Johann II.
Should Brazil be a monarchy, Prince Bertrand (77) would be the heir to his brother Pr.Luis.
Should Bavaria be a monarchy, Duke Max (81) would be the heir.
Provided that neither of the elder brothers would have abdicated
In all 3 cases, the heir is the sovereign's brother.
Charles and Gustav Adolf are the only heirs succeeding a very aged parent.
Gustav VI Adolf of Sweden succeeded to the throne of Sweden in 1950 aged 68.
In many Middle Eastern monarchies the succession went not from father to son but to the oldest male in the family. There may be some pretenders who are older than Charles is.
Had Grandduke Jean not abdicated his heir Henri would be getting close to Charles.
The Prince of Wales turns 70 years old today. Are there any European monarchies that have ever had an heir (apparent or presumptive) who was 70 or older?